The dimensions of fibres and in particular the cross-sectional dimensions transverse to the length of fibres are of considerable interest in determining the quality of fibres. In the case of fibres with an approximately circular cross-section the fibre diameter can be used to provide this information. For fibres with non-circular cross sections, a useful measure of the transverse ‘size’ of a fibre is its weight per unit length which is also known as linear density or fibre fineness. Fibre fineness is used widely with synthetic fibres where a variety of cross-sectional shapes can be readily engineered.
Average fibre fineness can be measured directly using a technique known as the gravimetric technique and is described in standard texts such as ‘Physical Properties of Textiles’, 2nd edition by W. E. Morton and J. W. S. Hearle. For a given sample:
                              Average          ⁢                                          ⁢          Fibre          ⁢                                          ⁢          Fineness                =                              (                          Total              ⁢                                                          ⁢              weight              ⁢                                                          ⁢              of              ⁢                                                          ⁢              fibre              ⁢                                                          ⁢              being              ⁢                                                          ⁢              tested                        )                                (                          Total              ⁢                                                          ⁢              length              ⁢                                                          ⁢              of              ⁢                                                          ⁢              fibre              ⁢                                                          ⁢              being              ⁢                                                          ⁢              tested                        )                                              [                  Equation          ⁢                                          ⁢          1                ]            
The gravimetric technique can be carried out by forming bundles of parallel fibres, cutting the bundles into known lengths, weighing the cut fibres, and then counting the number of fibres in the sample. The average fibre fineness of the fibres in the bundles is calculated as follows:
                              Average          ⁢                                          ⁢          Fibre          ⁢                                          ⁢          Fineness                =                              (                          Total              ⁢                                                          ⁢              weight              ⁢                                                          ⁢              of              ⁢                                                          ⁢              fibre              ⁢                                                          ⁢              being              ⁢                                                          ⁢              tested                        )                                              (                              Length                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                fibres                            )                        ×                          (                              Total                ⁢                                                                  ⁢                number                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                fibres                            )                                                          [                  Equation          ⁢                                          ⁢          2                ]            
This direct approach when undertaken manually requires considerable skill, time and labour. This is compounded by two practical limitations. Firstly, when using conventional weighing balances, the requirement of a certain level of precision in the weight measurement generally defines a minimum weight leading to a large number of fibres to be counted. Secondly, due to the inherent variability in many textile fibre samples and the need to obtain a measurement representative of the whole sample, it is generally necessary for the sample to have a large number of fibres.
Numerous attempts have been made to devise alternative methods for measuring the quality of fibres. The most successful alternative has been the development of porous plug or air flow fineness tester. The underlying principle of this method is that the resistance to airflow through a randomised compressed plug of fibres of known mass provides an indication of size of the pores between the fibres and thus the surface area of the fibres. Based on a theoretical relationship between the measured airflow resistance and the surface area per unit volume of the fibre it is possible to estimate the fineness of the fibres. The theoretical relationship is based on the assumption that the fibres are solid and have regular cross sectional shape.
This airflow approach has been widely applied for testing cotton and provides a characteristic of the fibres known as the ‘micronaire’ value. However, cotton and other cellulosic fibres generally have a central lumen and have irregular cross sectional shapes which complicates the interpretation and usefulness of the ‘micronaire’ value.
In cotton and other cellulosic fibres, the lumen or hollow centre is a result of the growth pattern of the fibre: layers of cellulose are deposited on the inside of the thin primary cell wall. FIG. 1 attached at the back of this specification illustrates the cross section of a cotton fibre.
For fibres having the structure shown in FIG. 1, another important property effecting the quality of the fibres is the degree of thickening of the cell wall which is sometimes referred to as fibre maturity. Following the nomenclature first introduced in the text “The Fineness and Maturity of Cotton” by F. T. Pierce and E. Lord (1939), the degree of thickening or circularity θ is defined from a cross section of a fibre to be the ratio of the wall area A to the area of a perfect circle having the same perimeter P:θ=4πA/P2  [Equation 3]The maturity value M of a fibre may be expressed as function of the degree of thickening as follows:M=θ/0.577  [Equation 4]
An empirical relationship between micronaire (Mic), fibre fineness(F) and maturity (M) was published in a journal article entitled “Airflow through Plugs of Textile Fibres Part II. The Micronaire test for cotton” by E. Lord in 1956. The empirical relationship is as follows:F*M=3.86*Mic2+18.16*Mic+13  [Equation 5]
Limitations of the micronaire value are readily apparent from Equation 5. In particular, micronaire is related to the product of fibre fineness and fibre maturity. As a result, a coarse immature sample of fibres and a finer more mature sample of fibres can both have the same micronaire value. Moreover an infinite set of fibre fineness and maturity values can provide the same micronaire value. In other words micronaire is not a comprehensive indicator of the quality of fibres.
One attempt at resolving this shortcoming of micronaire values was addressed by the so-called double compression airflow method. Specifically, rather than taking one airflow resistance measurement, the double compression airflow method involves taking measurements of two different compression ratios applied to the plug. The double compression airflow method is commercially available via at least two avenues namely, the Shirley fineness-maturity tester (FMT) and the ASTM test method D3818-1979.
An alternative method previously proposed by one of the inventors of the present invention was mentioned at the Beltwide Cotton Conference in 1999 and described in a paper entitled “Measuring Cotton Fineness Independently of Maturity Using the Sirolan Laserscan,” by G. R. S. Naylor and J. Sambell. The aim of the method was to semi-automate the direct gravimetric method for determining fibre fineness. The method involved suspending fibre snippets of approximately 2 mm long in a very dilute concentration in a liquid transport medium and counting the snippets one by one as each fibre snippet passed an optical sensor in the measurement cell. This approach utilised an existing instrument called the Sirolan-Laserscan as the automatic counter. Whilst this approach was found to give satisfactory results technically, it was too slow to be viable commercially.
At the Beltwide Cotton Conference in 2001 a paper entitled “Cotton Maturity and Fitness Measurement using the Sirolan-Laserscan” by G. R. S. Naylor identified that the fibre fineness value and a micronaire value independently measured could be used with Equation 5 to provide the average fibre maturity.
In addition, International patent application PCT/CH91/00017 (WO91/11705) describes the approach whereby fibres are placed in a plane between a lighting source and receiving optics connected to a CCD sensor i.e. a digital camera. The image is then manually analysed.
Several other approaches have been explored for determining cotton fineness. However, in spite of considerable research effort over the last 50 years there is still a need for a commercially viable method and apparatus for testing fibre fineness.